The present invention relates to photosensitive microcapsules and, more particularly, to an improved method for producing microcapsules wherein a synthetic viscosity modifier is incorporated in the continuous phase. Particularly useful synthetic viscosity modifiers include crosslinked polymers of acrylic acid which provide the desired degree of predictable viscosity control such that the proper particle size can be achieved during emulsification and the viscosity of the finished coating can subsequently be reduced to facilitate coating.
Imaging materials employing photosensitive microcapsules are the subject of commonly assigned U.S. Pat. Nos. 4,399,209 and 4,440,846. In the aforesaid commonly assigned U.S. patents, images are formed by image-wise exposing a layer of photosensitive capsules to actinic radiation and rupturing the capsules typically by passing the imaging sheet containing the capsules through a pressure nip. The radiation sensitive composition contains a photohardenable or photosoftenable material which undergoes a change in viscosity upon exposure. For example, in the most typical embodiments, the radiation sensitive composition contains a polyethylenically unsaturated monomer which polymerizes upon exposure, thereby causing the phase internal to the capsules to become harder. Due to the difference in the hardness of the capsules in the exposed versus the unexposed areas, only certain capsules rupture and release their contents. If the internal phase contains a dye precursor, the precursor is image-wise released, and a color image is formed upon its transfer to a developer layer. In previously disclosed embodiments, the developer layer may be present on the same support as the layer of capsules or a separate support. It is advantageous if the developer is present on the same support since such a self-contained imaging sheet can be developed as an integral unit.
An image transfer system in which the developer material is coated on a separate substrate as a separate developer or copy sheet is disclosed in commonly assigned U.S. Pat. No. 4,399,209. A self-contained imaging sheet in which the encapsulated color former and the developer material are co-deposited on one surface of a single substrate as one layer or as two interactive layers is disclosed in commonly assigned U.S. Pat. No. 4,440,846. An imaging system comprising a support, a layer containing microcapsules, a layer of developer material, and a layer containing an opacifying agent is disclosed in commonly assigned U.S. Pat. No. 4,766,050. The opacifying agent can form a separate layer or can be part of the layer containing the microcapsules or both but is interposed between the microcapsules and the developer to hide the microcapsules when viewing the image. U.S. Pat. No. 5,783,353, commonly assigned, discloses a self-contained imaging system wherein the imaging layer is enclosed between two supports to form an integral unit. The laminated format is advantageous in that it can reduce oxygen and moisture permeation and improve stability of the media. U.S. patent application Ser. No. 09/761,014, filed Jan. 16, 2001, discloses a self-contained photosensitive material which includes an imaging layer of photosensitive microcapsules and a developer on a support and a protective coating on the imaging layer.
Previously, photosensitive microcapsules having a desired uniform small size have been obtained by using pectin as a system viscosity modifier alone but preferably in combination with sulfonated polystyrene. This method is the subject of commonly assigned U.S. Pat. No. 4,962,010. Pectin is derived from natural sources and is, therefore, subject to significant variations. Controlling the viscosity and, consequently, the particle size of the microcapsules, can depend on the grade and even the batch of pectin being used. Furthermore, prior to coating, the viscosity of the microcapsule-containing coating composition must be reduced to levels suitable for the particular coating process. Viscosity reduction of coating compositions containing pectin-based viscosity modifiers depends on alkali deesterification of the pectin. Sodium hydroxide is typically used for this process and can result in a high pH medium which can interfere with development of the encapsulated color formers during image formation. Furthermore, the concentration and amount of alkali required to achieve the desired viscosity level depends on the degree of esterification of the pectin, which, as noted above, can vary significantly.